Sputtering, also known as physical vapor deposition (PVD), is a method of forming metallic features in integrated circuits. Sputtering deposits a material layer on a substrate. A source material, such as a target, is bombarded by ions strongly accelerated by an electric field. The bombardment ejects material from the target, and the material then deposits on the substrate. During deposition, ejected particles may travel in varying directions, rather than generally orthogonal to the substrate surface, undesirably resulting in overhanging structures formed on corners of high aspect ratio features in the substrate. Overhang may undesirably result in holes or voids formed within the deposited material, resulting in diminished electrical conductivity of the formed feature. Higher aspect ratio geometries have a higher degree of difficulty to fill without voids.
Controlling the ion fraction or ion density reaching the substrate surface to a desired range may improve the bottom and sidewall coverage during the metal layer deposition process (and reduce the overhang problem). In one example, the particles dislodged from the target may be controlled via a process tool such as a collimator to facilitate providing a more vertical trajectory of particles into the feature. The collimator provides relatively long, straight, and narrow passageways between the target and the substrate to filter out non-vertically travelling particles that impact and stick to the passageways of the collimator.
Attempts have been made to ignite a plasma in the PVD chamber at a low power level (e.g., about 100 W to about 3000 W) using less processing gas and/or lower gas pressures than used during processing of the substrate. However, the inventors have observed that conventional chambers have proven to be unreliable in igniting plasmas under such conditions. Specifically, previous ignition methods have proven to be unreliable, often requiring 10 or more attempts to ignite the plasma (e.g., ignition retries). Since excessive high voltage (e.g., 300V-3000V) or complicated waveforms may be used to breakdown the relatively higher pressure gases (much higher pressure than the high power deposition step), a high number of ignition retries could lead to defects or particle related issues in deposited films. The delay and uncertainty of ignition also may cause issues with the timing of process sequences and/or throughput issues in multi sequential processes on a complicated multi chamber semiconductor tool system. The inventors thus believe that limiting the number of ignition retries, ideally to be less than 2-3 times per wafer process at most, is very advantageous. However, such a retry limit would lead to a process fault using conventional techniques due to the ignition problems with previous ignition methods discussed above.
Thus, the inventors have provided improved methods for igniting a plasma, including igniting a plasma at low power and/or low pressure levels.